Many commercial processes for the production of high molecular weight polyester polymers of terephthalic acid and an ethylene glycol employ an antimony catalyst. The use of an antimony catalyst presents some problems when the polymer is melt spun into fibers, i.e., the antimony catalyst residues form deposits around spinneret holes. These deposits must be wiped off from time to time or the filaments will be defective. It is believed that the antimony deposits form because antimony is present in the polymer as antimony glycollate, which boils at about the temperature of the spinneret. The antimony glycollate vaporizes, and then hydrolyzes, leaving a non-volatile antimony containing deposit at the spinneret. A non-antimony containing catalyst is, therefore, a desirable goal.
High molecular weight polyester polymers of terephthalic acid and ethylene glycol are usually produced by one of three different processes, namely, (1) the direct esterification and then polymerization of pure terephthalic acid and ethylene glycol. (This direct polymerization process is satisfactory only if the ingredients are of high purity.) (2) Transesterification of dimethylterephthalate (DMT) and ethylene glycol to form, what is called in the art, "DMT monomer." The transesterification usually takes place in the presence of a manganese, zinc, or lithium catalyst. The DMT monomer is a mixture of ester products (low molecular weight polymer) in which most of the end groups are glycoxyl, and the mixture contains molecules with a degree of polymerization that is 1, 2, or 3. The DMT monomer is then further polymerized to high molecular weight polymer often with an antimony catalyst. The manganese catalyst used in the transesterification reaction tends to cause undesirable color in the final product unless it is sequestered; so phosphorus compounds, for example, H.sub.3 PO.sub.4, are often added before the DMT monomer is polymerized to sequester the manganese catalyst residue. (3) Reaction of medium purity (polymer grade) terephthalic acid and ethylene glycol to form, what is called in the art, "oligomer." The oligomer mixture contains color forming catalyst residues from the manufacture of terephthalic acid. These catalyst residues include manganese, iron, and molybdenum. These catalyst residues are then sequestered by the addition of a phosphoric compound, for example, H.sub.3 PO.sub.4, and then polymerized (polycondensed) to form high molecular weight polymer. The oligomer has a degree of polymerization of about 7, has 500 to 1000 carboxyl end groups per 10 to the 6 grams of oligomer. Some cyclic materials are present in the oligomer. (This third process does not require the highly purified starting materials necessary for the first process described above.) The catalyst of the invention is useful in all three of these processes. In the first process described, the catalyst of the invention may be added at the direct esterification step or at the beginning of the polymerization; in the second process described, the catalyst will be added after the DMT monomer is formed and after the manganese catalyst is sequestered (deactivated); and in the third process the catalyst will be added after the color-forming metal impurities in the oligomer are deactivated.
Combinations of metals compounds have been employed in the prior art as catalyst for the preparation of high molecular weight polymers of terephthalic acid and ethylene glycols: see, for example, Nichols U.S. Pat. No. 5,166,311; Engle-Bader et al. U.S. Pat. No. 5,116,938; and Kosky et al. U.S. Pat. No. 4,590,259.